Group A streptococci are responsible for a wide variety of human diseases, the most common of which are nasopharyngitis and impetigo. Nearly all clinical isolates have the antiphagocytic factor, M protein, on their surface. This virulence factor displays extreme antigenic diversity within its amino-terminal region. It is these highly variable portions of M proteins which form the basis of the serological typing scheme which was formulated in the 1930s prior to knowledge of any structural detail (Lancefield, R. C., J. Immunol. 89:307-313 (1962)).
In recent years, the sequences of M or M-like proteins have been reported (Frithz, E., Heden, L. O. and Lindahl, G. Mol. Microbiol. 3:1111 (1989)). The M-like molecules are structurally similar to M proteins in that they exhibit significant levels of sequence homology; however, they are not considered to be M protein itself because an antiphagocytic property has not been formally demonstrated.
Group A streptococci can be divided into two major classes partly on the basis of their immunoreactivity with a pair of monoclonal antibodies directed to epitopes which lie within the relatively conserved half of M proteins (Bessen, D. and Fischetti, V. A. J. Exp. Med. 172:1757 (1990); Bessen, D., Jones, K. F. and Fischetti, V. A. J. Exp. Med. 169:269 (1989)). Class I isolates are defined as those binding one or both monoclonal antibodies, whereas class II isolates do not bind either monoclonal antibody. In addition, the classes differ in their ability to exhibit opacity factor activity, and in several pathogenic properties of these organisms (Bessen, D. and Fischetti, V. A. J. Infect. Dis. 161:747 (1990)). For example, nearly all serotypes found in association with major outbreaks of rheumatic fever are class I. The classes also differ in ability to bind IgA, with this activity being specific for class II. Obviously this property is important to the function of streptococci as the arp4 protein reported by Lindahl et al. (European Patent Application 367890), e.g., is from class II streptococci and the IgA-binding protein of Russell-Jones et al. (U.S. Pat. No. 4,757,134) is from group B streptococci.
Immunoglobulin A is an important component of the bodily response to pathogens and other disorders. An early mucosal immune response to invasion is production of antibodies of the A class and as a result, elevated levels of IgA can be found in extracts of infected membranes such as saliva, urine, feces and urogenital extracts. Certain disorders such as kidney or liver malfunctions and early cancer detection may also be correlated to IgA. Indeed the human body produces more IgA daily than any other antibody class.
Accordingly, reagents to capture and measure IgA have been sought.